This invention relates to a photographic resin coated paper which comprises a paper or synthetic paper substrate at least one surface of which is coated with a resin composition comprising at least a titanium dioxide pigment and a resin. More particularly, it relates to a photograhic resin coated paper excellent in surface quality with substantially no micro-grits on the coated resin surface.
The term "micro-grits" used in this specification means fine materials or particles which appear on the surface of the coated resin in the photographic resin coated paper comprising a paper or synthetic paper substrate at least one surface of which is coated with a resin composition.
There are many causes for formation of the micro-grits. For example, causes for formation of micro-grits which appear on photographic resin coated papers made by melt extrusion coating of a thermoplastic resin on at least one surface of a paper or synthetic paper substrate by a melt extruder are as follows: (1) Use of thermoplastic resins which are apt to form gel; (2) When molten resin is extruded in a form of film through a die of melt extruder, proper temperature cannot be kept to result in a ununiform flow; (3) Stain of screen provided on breaker plate in the melt extruder, (4) Cracks in barrel liner of the melt extruder, (5) Insufficient kneading in the melt extruder, etc. However, in many cases, these are relatively easily solved by those skilled in the arts.
However, the most difficult is to prevent the formation of micro-grits on photographic resin coated papers comprising a paper or synthetic paper substrate coated with a resin composition comprising at least a thermoplastic resin and a pigment.
Usually, a pigment is incorporated in a thermo-plastic resin, preferably a polyolefin resin by a method which comprises previously preparing the so-called master batch containing a pigment in a high concentration and diluting it to the desired proportion with diluting resins or a method which comprises making the co-called compound by incorporating the pigment in the resin at the desired proportion from the first.
However, when the master batch or the compound is prepared by melt kneading a thermoplastic resin and a pigment by using the common melt kneading devices, especially, such as Banbury mixer and kneader, it often occurs that relatively coarse pigment particles or agglomerate of titanium dioxide particles are not finely dispersed in the thermoplastic resin, but are dispersed as they are, in the form of relatively coarse pigment particles or agglomerate of titanium dioxide particles. Thus, there are coarse pigment particles in the master batch or the compound. As a result, micro-grits are formed on the resin surface of photographic resin coated paper comprising a paper or synthetic paper substrate at least one surface of which is coated with a resin composition of pigment and resin prepared using said master batch or compound.
Significant photographic problems are brought about if micro-grits are present in resin coated papers used as photographic supports. That is, when a person is photographed on a photographic paper having a resin coated paper having micro-grits as a photographic support and if the micro-grits appear in the face of the person, commercial value of such photographs is completely lost.
It is well known that titanium dioxide pigment is generally used as a pigment contained in a resin layer on emulsion layer side of resin coated paper as a photographic support because of it's excellent whiteness, hiding power, resolving power, etc.
Photographic resin coated papers are also known. For example, those which comprise a paper substrate both surfaces of which are coated with a polyolefin resin, especially, polyethylene resin, the polyolefin resih layer on the emulsion coated side containing titanium dioxide pigment, colored pigment, fluorescent brighter, etc. are well known and disclosed in U.S. Pat. No. 3,501,298. Furthermore, as disclosed in Japanese Patent Unexamined Publication (Kokai) No. 30830/82, there are known photographic resin coated paper which comprise a paper substrate coated with a resin composition which is hardened with polymerization by electron radiation and which contains a white pigment such as titanium dioxide.
However, when the photographic resin coated papers which comprise a paper or synthetic paper substrate at least one surface of which is coated with a resin composition comprising at least a titanium dioxide pigment and a resin are produced using the master batch or compound of the titanium dioxide pigment as mentioned above, there has been the serious problem that micro-grits are apt to be formed on the resin surface of the resin coated papers due to coarse particles of pigment, titanium dioxide pigment in this case.